1. Field of the Invention
The present invention relates to optical telecommunication systems and, in particular, to devices for combining, splitting, and isolating light beams and methods for making the same.
2. Technical Background
Three port combining and splitting packages are widely used in local and long distance optical telecommunication networks. These networks comprise various polarization combining and splitting assemblies as part of Raman amplifiers and to increase the number of channels in a system. The necessity to design reliable optical devices for such systems, which are subject to various thermal and mechanical loads during their 20 to 25 year lifetime, is of significant importance. A typical example of such optical devices is a polarization-splitter package. A typical beam-splitter package comprises two assemblies. One assembly comprises one input single-mode (SM) optical glass fiber inserted into a single-capillary ferrule to produce a fiber-ferrule sub-assembly, a collimating lens, and a prism. A variety of optical blocks such as such as combiners, combiner-isolators, splitter-isolators, isolators, and the like are also substituted for the prism to form other useful devices. The optical components of this single-mode fiber assembly are embedded into an insulating glass tube, which in turn is mechanically protected by a metal housing. In a typical 3-port package the above single-mode fiber beam-splitting assembly is combined with an output collimating assembly leading to a pair of polarization-maintaining (PM) optical fibers. Beam-splitting packages are expensive and represent a significant cost in a typical communication system. Further, beam-splitting packages typically exhibit insertion losses higher than desired, resulting in degraded overall performance of the communications system or module. The problem is particularly acute during exposure to ambient operating conditions where temperature is variable.
Typical input glass ferrules enclosing the single-mode fibers employ a single capillary with relatively short (0.7-1.2 mm) fiber-receiving conical lead-in ends. With such input ferrules, the optical fiber is subjected to an S-bending over the short conical end portion which typically exceeds 50% of the fiber diameter (for a fiber having a 125 xcexcm diameter) on a span of about 6 to 10 diameters in length. This excessive micro bending increases the insertion losses. Fiber-ferrule subassemblies employing such ferrules are manufactured by inserting the optical fiber stripped of its polymer coating into the ferrule capillary; epoxy bonding the fiber into the ferrule capillary, including the conical end portion; grinding and polishing an angled facet on the fiber-ferrule; and depositing on the polished surface an anti-reflection (AR) coating. Once finished, the fiber-ferrule is aligned and assembled with the collimating lens and then embedded into the insulating glass tube, which, in turn, is protected by a metal housing.
There are two different technical solutions used in the design of bonds securing the components of an optical assembly. A low compliance bond between thermally well matched glass fibers and the glass ferrule is an approach commonly used by some manufacturers. The adhesives used are heat-curable epoxies with high Young""s modulus (E greater than 100,000 psi) and moderate to high thermal expansion coefficients (xcex1=40-60 10xe2x88x926xc2x0 C.xe2x88x921). A typical example would be 353 ND EPO-TEK epoxy adhesive. In addition, the bond thickness used is very small.
Silicon adhesives are used to bond thermally mismatched glass tubes with metal housings and glass optical elements with metal holders. In these joints, a high compliance design is used. The silicones, which can be cured between 20-150xc2x0 C. in the presence of moisture, are typically characterized by an extremely low Young""s modulus (E less than 500 psi) and high thermal expansion (xcex1=180-250 10xe2x88x926xc2x0 C.xe2x88x921). A typical example would be DC 577 silicone, which can be used to bond, for example, a metal optical filter holder to a collimating lens.
Adhesive bonding with subsequent soldering or welding is used to encapsulate a polarization-splitting assembly into a three-port package. Such a polarization-splitting package enclosure, which is typically formed of six to eight concentric protective units, has micron transverse tolerances. Maintaining these tolerances requires precision machining and may require time-consuming alignment and soldering with frequent rework. As a result of these limitations, the optical performance specifications are lowered and cost is increased. As an example, soldering may include several re-flow cycles. This induces local thermal stresses in the nearby adhesive bonds and leads to the degradation of the polymer adhesive which can result in repositioning of optical components and a shift in the optical performance. With such design, soldering may also result in the contamination of optical components through direct contact with molten solder and/or flux.
However, it is desirable to obtain a high accuracy thermally compensated optical multiple-port package that can be relatively inexpensive, reliable, and have a low insertion loss. Additionally, a package design should be adequate not only to mechanically protect the fragile optical components but also to compensate for and minimize the thermally induced shift in optical performance. Further, it is desirable to obtain a multiple-port package, such as six port packages, with the same qualities since they further reduce costs and reduce size. Thus, there exists a need for such optical packages and a process for manufacturing such optical packages, which is miniaturized, has a low insertion loss, is inexpensive to manufacture, and which results in a device having reliable, long-term operation.
The present invention provides a dual polarization combiner-splitter package and provides a method of manufacturing the package from components such as input ferrules, collimating lenses (e.g. aspheric lens), optical fibers, and prisms, utilizing bonding adhesives in a manner which allows the alignment of the individual components relative to one another with a precision and a manufacturability that makes it possible to produce commercial devices having six or more ports. This had heretofore not been achieved. In one aspect, the invention includes an improved input ferrule and prism holder which permits bonding through the utilization of UV and thermally curable adhesives and improved thermal curing to reduce relevant internal stresses in the assembly so formed. For assemblies having multiple pairs of fibers (e.g., six port devices) the invention also provides improved fiber ferrule designs and manufacturing methods for devices that have low IL, operate over a wide temperature range, are reliable, and cost effective.
In one aspect of the invention, improvements to fiber ferrules are provided including capillary designs and tolerances. The invention provides designs for capillaries which resist movement of the optical fibers during adhesive curing, soldering, welding, and environmental thermal changes. One technique uses washers to precisely position optical fibers in a capillary. Yet another aspect of the invention is the selection of optical fibers based on geometric properties such as: outer (cladding) diameter, circularity of the cladding (ovality), and core concentricity. In another aspect, the invention teaches matching the separation distance (SD) between optical fibers on each end of the package. Tolerances for the separation distances are provided which make possible the commercial manufacturability of six-port devices.
Methods embodying the present invention include the steps of providing ferrules with capillaries having certain shapes and satisfying predetermined tolerances for the walls of the capillaries, providing single-mode optical fibers and polarization mode optical fibers satisfying predetermined tolerances for outer diameter, ovality, and centricity, providing prism holders, bonding the prism to the holder with liquid adhesive, bonding the prism holder to one of the collimating lenses, and aligning the single-mode fibers with the polarization mode fibers. In an embodiment of the invention, the subassembly is subsequently thermally cured through an accelerated dark cure sequence followed by a final high temperature curing. In another embodiment of the invention, UV radiation is applied to the prism holder/prism interface. The UV light source may be dithered such that UV radiation uniformly covers the cylindrical interface between the prism holder and the outer surface of the collimating lens.
In a preferred method of manufacturing the invention, subsequent to the UV curing process, the assembly is cured through a stress relaxation cycle at about 40-50xc2x0 C. for two to four hours followed by a thermal curing cycle of about 95 to 110xc2x0 C. for one to two hours.
In one embodiment of the invention, the ferrules are employed with an input cone having an axial length greater than about 2.5 mm to reduce S-bending of input fiber, thereby minimizing resultant insertion losses. In another embodiment of the invention, a generally cylindrical filter holder has an annular seat formed in one end for receiving a prism and a lens-receiving aperture at an opposite end having an internal dimension of sufficient dimensions to enclose a portion of a collimating lens. The preferred prism holder is made of glass having a coefficient of thermal expansion similar to the adjacent collimating lens. The prism holder may also be made of suitable metals preferably having similar thermal expansion characteristics. One embodiment for the prism holder includes slots or openings in the lateral surface such that UV light enters and cures adhesive between the lens and prism holder. An optical splitting assembly of a preferred embodiment of the present invention includes such an improved ferrule enclosing two single-mode fibers and a prism holder coupled in alignment with another ferrule enclosing four polarization-maintaining fibers and both assemblies enclosed in a suitable housing.
The methods and apparatus described herein facilitate the manufacture of a six-port optical device which results in several advantages. For example, in a six-port device having two single-mode optical fibers in the input collimating assembly coupled through a prism to two pairs of polarization-maintaining fibers operates with two transmitted light beams. The two beams are split into four polarized beams which are transmitted to the polarization-maintaining fibers. The package thereby functions as two polarization beam splitters in a single package and thus reduces by half the number of prisms, collimating lenses, ferrules, and enclosure units. Thus, for example, the same six-port combiner-splitter package can be used in communications systems and laser applications and reduce by one half the number and also the size of the packages needed as compared to three-port packages.
The manufacturing method and optical element assembly of the present invention, therefore, provides an improved performance optical assembly utilizing unique ferrules, prism holder, and an assembly method for providing a low cost, highly reliable, and improved performance combiner-splitter package and using these packages in optical modules which can be used in various optical communications system.
The devices of the instant invention are applicable for packages that comprise similar optical devices including crystal-based isolators, circulators and the like.
Additional features and advantages of the invention will be set forth in the detailed description which follows and will be apparent to those skilled in the art from the description or recognized by practicing the invention as described in the description which follows together with the claims and appended drawings.
It is to be understood that the foregoing description is exemplary of the invention only and is intended to provide an overview for the understanding of the nature and character of the invention as it is defined by the claims. The accompanying drawings are included to provide a further understanding of the invention and are incorporated and constitute part of this specification. The drawings illustrate various features and embodiments of the invention which, together with their description serve to explain the principals and operation of the invention.